Keisuke Furumitsu

Food habits of small fishes in seagrass habitats in Trang, southern Thailand

To clarify the feeding habits of seagrass fishes, we examined the gut contents from 42 fish species collected in seagrass habitats in Trang. Thirteen species showed ontogenetic and/or seasonal changes in food-use patterns. Smaller individuals generally preyed on small planktonic items (e.g., copepod larvae) or small benthic/epiphytic crustaceans (e.g., harpacticoid copepods), with subsequent changes to other prey items (e.g., shrimps, crabs, detritus and filamentous algae) with growth. The most important dietary items for the seagrass fish assemblages comprised benthic/epiphytic crustaceans, detritus, and planktonic copepods. Cluster analysis based on dietary overlaps showed that the seagrass fishes comprised eight feeding guilds (large benthic/epiphytic crustacean, detritus, planktonic animal, small benthic/epiphytic crustacean, mollusc, invertebrate egg, polychaete, and fish feeders). Of these, the first three guilds were the most abundantly represented, whereas the last three were each represented by only a single species.

A new species of eagle ray Aetobatus narutobiei from the Northwest Pacific: an example of the critical role taxonomy plays in fisheries and ecological sciences.

Recent taxonomic and molecular work on the eagle rays (Family Myliobatidae) revealed a cryptic species in the northwest Pacific. This species is formally described as Aetobatus narutobiei sp. nov. and compared to its congeners. Aetobatus narutobiei is found in eastern Vietnam, Hong Kong, China, Korea and southern Japan. It was previously considered to be conspecific with Aetobatus flagellum, but these species differ in size, structure of the NADH2 and CO1 genes, some morphological and meristic characters and colouration. Aetobatus narutobiei is particularly abundant in Ariake Bay in southern Japan where it is considered a pest species that predates heavily on farmed bivalve stocks and is culled annually as part of a ‘predator control’ program. The discovery of A. narutobiei highlights the paucity of detailed taxonomic research on this group of rays. This discovery impacts on current conservation assessments of A. flagellum and these need to be revised based on the findings of this study.

Age, growth and age at sexual maturity of fan ray Platyrhina sinensis (Batoidea : Platyrhinidae) in Ariake Bay, Japan

Age, growth and sexual maturity of the fan ray Platyrhina sinensis in Ariake Bay, Japan were determined from specimens collected from May 2002 to September 2006. Age determination was conducted by vertebral centrum analysis using soft X-radiography. Annual band pair deposition was determined by marginal increment and edge analyses. The von Bertalanffy growth model best described the overall pattern of growth for both males and females (males L∞=455.2, k=0.56, t0=−1.09; females L∞=555.8, k=0.28, t0=−1.77; L∞ is the theoretical asymptotic total length in mm, k is the growth rate coefficient and t0 is the theoretical time at zero length). Parameter estimates suggest that females attain a larger asymptotic total length and grow more slowly than males. The observed maximum ages were 5 years for males and 12 years for females. Age at 50% sexual maturity was 2.1 years for males and 2.9 years for females. The results indicate that this species is relatively fast-growing, short-lived and early maturing compared with many batoid species.

Mitochondrial genome of Japanese angel shark Squatina japonica (Chondrichthyes: Squatinidae)

Abstract Squatina japonica belonging to the monogenetic family Squatinidae is endemic to the Northwest Pacific. The complete mitochondrial genome sequence of S. japonica is 16,689 bp long and comprises 13 protein-coding genes, 22 tRNA genes, 2 rRNA genes, and 1 control region. The base composition of the genome is 31.10% A, 31.04% T, 24.42% C, and 13.43% G. The geographic clade and phylogenetic relationship of S. japonica are ambiguous. Therefore, studying the complete mitochondrial genome of S. japonica is highly important to understand the aforementioned aspect and to analyze the conservation genetics in the genus Squatina.

Mitochondrial genome of longheaded eagle ray Aetobatus flagellum (Chondrichthyes: Myliobatidae)

Abstract The complete mitochondrial genome sequence of the Aetobatus flagellum is 20,201 bp long and consists of 13 protein-coding genes, 22 tRNA genes, 2 rRNA genes and 1 control region (CR). The base composition of the genome is 30.9% A, 28.2% T, 27.1% C and 13.8% G. Comparing mtDNA of elasmobranchs submitted in NCBI, our study not only identified the longest mitochondrial genome with 4490 bp CR in A. flagellum, but also strongly revealed that records in the northwest Pacific may belong to a separate species from those distributed in Indonesia.

Effects of habitat change from a bare sand/mud area to a short seagrass Halophila ovalis bed on fish assemblage structure: a case study in an intertidal bay in Trang, southern Thailand

Fish assemblage structures in an intertidal sand/mud area invaded by short seagrass Halophila ovalis and a nearby non-invaded sand/mud area in Trang Province, Thailand, were examined in detail by visual census to elucidate the effects of such habitat change on assemblage structure. The assemblage structure in the newly established seagrass bed showed a significant shift from that in the sand/mud area, despite the total fish species numbers remaining similar to each other (i.e., 30 and 29 species, respectively). Total fish density was significantly lower in the seagrass bed. In addition, differences in density patterns of component species between seagrass and sand/mud fish assemblages were evident, some fish species being restricted to or more abundant in the alternative habitat. Such differences may have arisen, at least in part, from differences in practical food availability/accessibility between the two habitat types, and/or specific microhabitat preferences of the resident fishes. Although seagrass habitats are often regarded as supporting a richer fish assemblage compared with bare sand/mud areas, the present study clearly indicated that the latter also supported a unique fish assemblage, including juveniles of fishery target species. Accordingly, both seagrass and bare sand/mud habitats should be taken into consideration for both the conservation of greater overall biodiversity in the coastal environment and the continued existence of local fisheries.

Dietary habits of the fanray Platyrhina tangi (Batoidea: Platyrhinidae) in Ariake Bay, Japan

The dietary habits of the fanray Platyrhina tangi were investigated by analyzing the stomach contents of specimens collected in Ariake Bay, Japan. Of 334 stomach specimens, 324 contained food and 10 (3.0%) were empty. The mean percentage weight of stomach contents per unit of body weight was 0.59%. Thirty-seven taxonomic levels of prey were identified. The most common prey was shrimp, followed by fish and mysids. There were no differences in the composition of the diet between sexes, but an ontogenetic dietary shift was observed. Shrimps were the most common prey in all size classes. In addition, smaller individuals frequently ate mysids, and larger individuals often consumed fish. Dietary breadth values increased with size. Trophic level analysis revealed that trophic level increased with size; however, this species is consistently a secondary consumer. Dental sexual dimorphism was also observed. Specifically, mature males had much longer and sharper cusps than females and immature males. Since males and females had similar diets, dental sexual dimorphism may be related to their reproductive behaviour.